1. Field of the Invention
The present invention relates to an electrode paste for a solar cell and a solar cell electrode using this paste.
2. Technical Background
Electrodes for gathering electrical current are formed in solar cells. An electrode paste containing an electrically conductive component is typically used to form the electrodes. Electrically conductive particles, glass frit and an organic medium are contained in the electrode paste. Various other additives are also contained as necessary.
Pb-based glass frit having various superior characteristics such as sintering characteristics has been used as glass frit. However, from the viewpoint of reducing the burden on the environment, frit has come to be required to be lead-free, and lead-free glass such as Bi-based glass frit is required to be used for the glass frit contained in electrode pastes.
Although electrode paste is fired after being coated onto a solar cell substrate, the firing conditions of lead-free glass tend to be subject to limitations. In other words, in the case of firing an electrode paste containing lead-free glass, the fireable temperature range tends to be narrow, making it difficult to set an optimum firing temperature in consideration of various conditions such as electrode performance, productivity and yield. In addition, in the case the fireable temperature range is extremely narrow; there is increased susceptibility to the occurrence of defective products in the case the firing temperature deviates from the set value for some reason.
Zinc oxide is known to be an additive for electrode pastes. For example, zinc oxide is added for the purpose of improving electrical characteristics and tensile strength in Japanese Patent Application Laid-open No. 2003-152200. Japanese Patent Application Laid-open No. 2005-243500 discloses a technology for adding zinc oxide having a mean particle diameter of 0.001 to 0.1 μm for the purpose of improving electrical continuity and adhesive strength. Japanese Patent Application Laid-open No. 2006-302890 discloses a technology for adding zinc oxide of 7 to 100 nm.
A need exists to provide an electrode paste for use in solar cell manufacture that has a wide range of firing temperature conditions.